When heartburn strikes, an antacid tablet (a base) settles your stomach acid in minutes. When a truck spills acid on a highway, crews spread a basic powder over it before cleaning up. When lakes near Sudbury were left almost lifeless by acid rain, part of the recovery came from cutting smelter emissions and, in some lakes, adding crushed limestone. All of these rely on one reaction, neutralization. This lesson shows how it works and then looks at how chemical reactions create environmental and safety problems, and how other reactions help solve them.
Neutralization: acid + base → salt + water
In a neutralization reaction, an acid reacts with a base to produce a salt and, usually, water. The heart of the reaction is the hydrogen ion from the acid combining with the hydroxide ion from the base: . The leftover ions (the base's metal ion and the acid's non-metal or polyatomic ion) make up the salt. Chemists use "salt" for any ionic compound formed this way, not just table salt. Because the ions swap partners, neutralization is a kind of double displacement. Mixing the right amounts brings the pH close to 7 (exactly 7 only for a strong acid with a strong base).
| Acid | Base | Balanced equation | Salt |
|---|---|---|---|
| hydrochloric acid | potassium hydroxide | HCl + KOH → KCl + H₂O | potassium chloride |
| nitric acid | sodium hydroxide | HNO₃ + NaOH → NaNO₃ + H₂O | sodium nitrate |
| sulfuric acid | potassium hydroxide | H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O | potassium sulfate |
| nitric acid | calcium hydroxide | 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O | calcium nitrate |
| hydrochloric acid | magnesium hydroxide (an antacid) | 2HCl + Mg(OH)₂ → MgCl₂ + 2H₂O | magnesium chloride |
Chemistry that solves environmental problems
Burning coal and smelting metal ores that contain sulfur release sulfur dioxide, ; vehicles and power plants release nitrogen oxides. In the air these gases react with oxygen and water to form sulfuric, sulfurous and nitric acids, which fall as acid rain (rain below about pH 5.6). Acid rain lowers the pH of lakes, harming fish and other aquatic life, damages forests and soils, and dissolves limestone and marble buildings and statues. For example: , and followed by .
- Neutralizing acid spills: emergency crews spread a weak base such as sodium hydrogen carbonate, sodium carbonate (soda ash) or crushed limestone on the spill until the fizzing stops, then collect the residue. Base spills are treated with a weak acid such as citric acid.
- Liming lakes: adding crushed limestone () to an acidified lake neutralizes some of its acid and raises the pH, buying time while emissions are reduced.
- Scrubbing smokestack emissions: a scrubber sprays a slurry of limestone or lime through the exhaust gases, capturing sulfur dioxide as solid calcium sulfate (gypsum), which can be used to make drywall. Sudbury's smelters have also cut their emissions dramatically since the 1970s, partly by capturing sulfur dioxide and turning it into sulfuric acid to sell.
- Renewing the Great Lakes: under the Great Lakes Water Quality Agreement, first signed by Canada and the United States in 1972, phosphates in laundry detergents were limited to reduce the algae blooms that had choked Lake Erie, and polluted harbours called Areas of Concern are being cleaned up. In Hamilton Harbour, the Randle Reef project has sealed coal-tar-contaminated sediment inside an engineered containment facility built in the harbour, which is being capped and turned into new port land.
Safety and environmental issues
| Issue | The chemistry | How the risk is reduced |
|---|---|---|
| Cyanide in gold mining | Gold is dissolved out of crushed ore with a dilute sodium cyanide solution. Cyanide is highly toxic to fish, wildlife and people; a tailings dam failure at Baia Mare, Romania, in 2000 sent cyanide-contaminated water into rivers flowing to the Danube. | Mines destroy cyanide in their wastewater before release, for example by reacting it with sulfur dioxide and air to form much less toxic cyanate (the SO₂/air process, developed by the Canadian nickel company Inco in the early 1980s), and they build and monitor stronger tailings dams. |
| Corrosion of bridge supports | Steel reinforcing bars inside concrete rust when water and oxygen reach them, and chloride ions from road salt speed this up. Rust takes up more space than steel, so it cracks the concrete from inside. Salt damage was a major reason Montréal's original Champlain Bridge was replaced by a new bridge that opened in 2019. | Epoxy-coated, galvanized (zinc-coated) or stainless-steel bars, sealed concrete, paint, and cathodic protection. In galvanized steel the zinc is more reactive than iron, so it corrodes first and protects the steel. |
| Chlorine in swimming pools | Chlorine compounds such as sodium hypochlorite form hypochlorous acid, which kills bacteria. It works best at pH 7.2 to 7.8. Sunlight breaks it down, and it reacts with sweat and urine to form chloramines, the cause of the strong pool smell and red eyes. | Testing and adjusting pH and chlorine levels often, adding a stabilizer for outdoor pools, and showering before swimming. |
| Bromine in hot tubs and indoor pools | Bromine compounds sanitize the same way but stay effective at higher temperatures and over a wider pH range, and their by-products are less irritating. | Used mainly indoors and in hot tubs because sunlight breaks bromine down and it cannot be stabilized; it also costs more than chlorine. |